Brucellae, Gram-negative coccobacilli, are facultative intracellular bacterial pathogens of both humans and animals, which cause brucellosis, a zoonotic disease that is difficult to diagnose and treat. It is characterized in humans by undulant fever, and if untreated, can develop into chronic infection with symptoms persisting for several month. Chronic infections may result in infection of secondary tissues, including heart and brain. Symptoms may also recur years after the original infection. The pathological manifestations of brucellosis include arthritis, endocarditis, and meningitis in humans, while animal brucellosis is characterized by spontaneous abortion. Brucella infection is acquired by humans through contact with infected livestock and consumption of unpasteurized dairy products. Additionally, Brucella can be used as a biological weapon since transmission through a spray is possible. The bacteria are highly contagious and it is suggested that 10 to100 bacteria would be sufficient to produce a contaminating spray for humans, and several countries are suspected to have studied the agent as a biological weapon Guihot A, et al, Bioterrorism with brucellosis. Presse Med 2004, 33: 119-22. Six species are recognized within the genius Brucella with the main pathogenic species worldwide being B. abortus, B. melitensis and B. suis. The common feature of Brucella species is the presence of lipopolysaccharide (LPS) in the outer membrane, which is considered a major virulence factor and a protective antigen It is composed of an unbranched homopolymer of 4,6-dideoxy-4-formamido-alpha-D-monnopyranose. Our approach is based on preparing protein conjugates of an O-specific polysaccharide (O-SP) isolated from LPS and cross-reactive with all three pathogenic species. We prepared conjugates of O-SPs bound to aminooxylated BSA. Conjugates were injected into mice and induced antibodies, with the booster responses, which reacted with both B. abortus and B. suis by Elisa. Additionally, we studied the chemical structure of the O-SP and core part linking O-SP to Lipid A. Our results show that the O-chain contains no repeating units of alternating 2- and 3-substituted Rha4N. Instead, a minor amount of 3-substituted Rha4N is present at a defined position at the non-reducing end. The quantity of these modification differs relative to the amount of chains built of uniformly 2-substituted Rha4N and can define serological differences between strains. The lipid part of Brucella LPS is not similar to traditional lipid A which has a disaccharide backbone and 3-hydroxy fatty acids and this explains the absence of the usual biological properties typical for lipid A. The arrangement of the structural fragments linking the O-chain to lipid A is being studied. Klebsiellae: In recent years many antibiotic resistant strains of Klebsiella pneumoniae have been isolated worldwide. A particular threat to the public health is the spread of carbapenemase producing K. pneumoniae strains (KPC) because of the limited options for treating infections. K. pneumoniae produces a capsular polysaccharide (CPS), a protective antigen used in commercially available bacterial vaccines, but which capsular serotypes are predominant in KPC strains is not known. We studied the LPS and CPS structures of a carbapenem-resistant K. pneumoniae clinical isolate from the 2011 outbreak at the U.S. National Institutes of Health Clinical Center. Analysis of the LPS revealed that it a rough variant with no O-specific polysaccharide and the chemical structure of two core oligosaccharide variants is in agreement with the published data. Analyses of the mucous CPS showed that it is composed of repeating units containing residues similar to several other serotypes, like galactose, rhamnose and uronic acid, but with linkages different from any of the published structures. We believe that knowing this novel CSP structure will facilitate fast diagnoses and help finding new therapeutic solutions to this antibiotic resistant pathogen.